Make rootfs-image-v2 user consumable

Acceptance criteria:

• Make partition numbers not hardcoded, for example by a source /etc/mender/rootfs-image-v2.conf
• Partition switching is not atomic, since the fw_setenv is done in three passes instead of one. It's a simple rewrite of those commands to do it in one pass instead.
• Reboot sleep should be ten minutes to match client
• Test manually successful update and rollback
• Article describing it on Mender Hub, as a community/unsupported module